Determination of Melting Points
Background Information
MELTING POINTS: Most organic solids melt at a specific temperature,
which gives us useful information to describe organic solids, just
like height is useful information to describe a person. And, just like
the height of a person, the melting point alone is not enough
information to know exactly what the solid is.
In this lab session, you will measure the melting points of urea and
trans-cinnamic acid, which melt at about the same temperature, and
then you will find out whether an unknown organic solid is urea or
trans-cinnamic acid and using a technique called mixture melting
point.
PUTTING THE SOLID IN A MELTING POINT CAPILLARY TUBE: Your TA will
demonstrate the proper way to pack the solid in the capillary tube.
Important notes:
(1) Use a small amount.
(2) Pack the solid well in the capillary tube.
(3) If the crystals aren't small, use a metal spatula to crush them to
a fine powder before putting a sample in the capillary tube.
MEASURING THE MELTING POINT: Your TA will demonstrate the proper way
to use the melting point apparatus
(4) Be sure the apparatus has cooled to 120°C or less before you place
your capillary in it when you're taking the melting point of pure urea
or pure trans-cinnamic acid.
(2) Be sure the apparatus has cooled sufficiently (check with your TA
about the temperature) before you place your capillary in it when
you're taking the melting point of a mixture of urea and transcinnamic acid or a mixture of your unknown and either urea or transcinnamic acid.
(3) The "melting point" is actually a melting range. Report the
temperature at which melting begins and at which melting is complete
as the melting range (e.g., 106.5 - 108°C). Raise the temperature
slowly in the vicinity of the melting range - about 2o/minute is
generally good.
(4) The melting point apparatus will let you determine the melting
points of more than one sample at a time. Do two or more together when
you think that they will melt at about the same temperature.
MIXTURE MELTING POINTS: Pure substances melt at defined temperatures,
but mixtures of organic compound melt over a range that is lower than
the melting point of each of the components. This gives you an easy w,
to see if two organic solids are the same substance: Mix some of each
and see if the mixture melts over a range that is lower than either of
the two components.
Health & Safety Hazards
Urea: Harmful. Possible risk of irreversible effects. Irritating to
eyes, respiratory system and skin. In case of contact with eyes, rinse
immediately with plenty of water and seek medical advice. Wear
suitable protective clothing.
Trans-Cinnamic acid: Irritant. Irritating to eyes, respiratory system,
and skin. In case of contact with eyes, rinse immediately with plenty
of water and seek medical advice. Wear suitable protective clothing.
Melting point apparatus. Portions become very hot and can cause
serious burns.
Procedure
1. Determine the melting points of pure urea and pure trans-cinnamic
acid.
2. Determine the melting range of a mixture (about 50:50) of urea and
trans-cinnamic acid.
3. Determine the melting point of your unknown.
4. Determine the melting range of a mixture (about 50:50) of your
unknown and urea and of a mixture (about 50:50) of your unknown and
trans-cinnamic acid.
Report: Determination of Melting Points
Your Name:_________________________ Your Lab Partner's Name __________
TA_____________
Lab Section (circle one and write the lab meeting time [8-11, 11-2, 2-5])
Tuesday
Results
Melting
Melting
Melting
melting
melting
melting
Thursday
point
point
range
point
range
range
of
of
of
of
of
of
urea: _______________
trans-cinnamic acid: _________________
mixture of urea and trans-cinnamic acid
unknown: ______________________
unknown mixed with urea: ______________
unknown mixed with trans-cinnamic acid: _____________
Conclusion
Identity of unknown: _________________
Questions: Determination of Melting Points
1. What is a safety hazard associated with this lab session?
2. What is a health hazard associated with this lab session?
3. Why should you pack the substance well in the melting point
capillary tube?
4. What two errors in the data will result if you raise the
temperature of the melting point apparatus too quickly?
5. Name an organic solid that does NOT have a specific, defined
melting point.